1. Field of the Invention
This invention generally relates to the inhibition of plugging in supercritical water reaction systems. More particularly, the invention relates to the inhibition of plugging in components upstream from a supercritical water reactor.
2. Description of the Related Art
Materials that are reacted in supercritical waste oxidization ("SCWO") systems typically operate at relatively high pressures and high temperatures (e.g., at least about 700.degree. F. and 3200 psia (about 370.degree. C. and 220 bar)). Other systems may operate in the vicinity of supercritical conditions for water (i.e., at least about 500.degree. F. and about 2000 psia (about 260.degree. C. and 138 bar). Inorganic salts tend to have a low solubility within supercritical water. At a constant pressure, the solubility of salts typically increases as the temperature of the water increases. As the temperature approaches within about 10.degree. F. (about 5.degree. C.) of the supercritical temperature of water, the solubility of inorganic salts tends to drop to low values. Concurrently, the solubility of organic compounds, normally relatively insoluble in water, increases. This increase in solubility tends to assist the oxidation of the organic compounds.
Oxidizable matter that is reacted or oxidized in SCWO systems may cause solids or char to form within the reactor due to the low insolubility of the oxidation products within supercritical water. Alternately, the streams themselves (especially hazardous waste streams) may contain salts or solids, which tend to be insoluble at supercritical conditions. The presence of salts or solids in the reaction zone of the reactor tends to cause problems. For instance, if significant amounts of salts or solids are generated, the salts or solids may either partially or fully plug the reactor, thereby reducing reactor efficiency and/or causing expensive reactor shut-downs for maintenance purposes. Given the particularly high temperatures and pressures at which these systems operate, the replacement and/or maintenance of equipment in such systems tends to be expensive. Therefore, plugging in these systems tends to be a particularly difficult problem to address.
The plugging problem may be accentuated if additives are mixed with the stream to be treated. For instance, additives may be mixed with a given stream to raise or lower its pH (e.g., for the purpose of reducing corrosion) or to neutralize corrosive elements in the stream. These additives, or compounds produced from the additives, may in turn cause plugging in the system. By way of example, if a stream has a low pH, a practitioner may wish to add sodium hydroxide (NaOH) to the stream to raise the pH. If the waste stream contains chlorinated compounds oxidation of these compounds will typically produce hydrochloric acid (HCl). To inhibit the corrosion due to HCl, as well as other acidic compounds, NaOH is typically added to neutralize these acidic products. The reaction of NaOH with HCl typically produces the salt sodium chloride (NaCl). The formed salt may cause plugging of the reactor. Thus the additives available to control stream pH, and/or system corrosion, have necessarily been limited by practical considerations related to system plugging.
During at least one treatment procedure water is heated to supercritical conditions before entering the reactor, typically before oxidants are added. For salt containing waste streams, the rise to supercritical conditions may cause precipitation to occur prior to reaching the reactor. Thus plugging of the system may occur within the heater and conduits upstream from the reactor. While a number of reactor designs are known to prevent plugging of the waste stream within the reactor, little has been done to prevent plugging in the heater or conduits leading to the reactor. It is therefore desirable to devise a method and system for preventing the precipitation of salts within the system upstream from the reactor due to the rise to supercritical conditions. Such a method may help prevent the plugging of the system.
Conventional reactor assemblies operating in the vicinity of supercritical conditions for water are described in detail in the following patents or patent applications:
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